Over the past 10 to 15 years the electric power industry and academia have conducted a number of research programs to identify those mechanisms that are responsible for the premature failure of underground electric cables. The results of these activities indicate that many cable failures can be linked to the internal build-up of water trees within the polymeric insulating layers of the cable. Water treeing can be described as micro sized "branching" that is similar to the branch pattern of a tree. These water filled imperfections branch radially inward through the amorphous insulating materials used in today's distribution cables. As the water treeing progresses radially inward the potential for cable failure increases.
Industry has tried to resolve this problem by developing new cable designs that provide superior protection to the infiltration of water. Unfortunately, existing designs that work well to mitigate this problem, such as metallic foils, substantially increase the cost of cable manufacturing.
Recent advances in polymer technology are providing new approaches for solving the water treeing problem described above. Thermotropic liquid crystal polymers (LCPs) are a new class of polymer that provide outstanding water barrier protection. LCPs have water vapor permeabilities that are two orders of magnitude lower than standard polymer materials used for jacketing electrical distribution cable.
LCPs are anisotropic materials that when processed into films form a laminar structure similar to pages in a phone book. These `submicron thick laminar sheets` lie parallel to the surface of the film and each layer forms a distinct and relatively non-interconnected barrier that resists the formation of continuous radial microcracks.
LCPs derive their outstanding properties from their rigid-rod molecular structure which at a macroscopic level results in self-reinforced materials with exceptional strength, stiffness and barrier properties. The high degree of molecular order of the LCP molecules allows them to attain a very tight packaging density (similar to logs in a river). When combined with the LCP's extremely low gas solubility, an ideal structure is formed for providing superior gas and liquid barrier properties.
Although thermotropic LCPs possess a variety of properties that make them an attractive candidate for this application, standard LCP processing techniques result in films with uniaxial orientation. Such films have exceptional machine (extrusion) direction mechanical properties and very poor transverse mechanical properties. If standard extrusion techniques are used to extrude LCPs over a tubular structure, the resulting LCP coating with its axial molecular orientation would readily split in the axial direction when exposed to even the slightest degree of bending.